Medical implant or medical implant part and method for producing the same

ABSTRACT

The invention provides a medical implant or medical implant part comprising a body and a surface layer, wherein the surface layer comprises a mixture comprising at least one hydrophilic polymer and ultrahigh molecular weight polyethylene having a weight average molecular weight of about 400,000 atomic mass units or more. The invention also provides a method for producing such a medical implant or medical implant.

FIELD OF THE INVENTION

This invention pertains to medical implants or medical implant partscomprised of ultrahigh molecular weight polyethylene and methods ofproducing and using the same.

BACKGROUND OF THE INVENTION

Ultrahigh molecular weight polyethylene (“UHMWPE”) is commonly used inmaking orthopaedic implants, such as artificial hip joints. In recentyears, it has become increasingly apparent that tissue necrosis andosteolysis at the interface of the orthopaedic implant and the host boneare primary contributors to the long-term loosening failure ofprosthetic joints. It is generally accepted by orthopaedic surgeons andbiomaterials scientists that this tissue necrosis and osteolysis is due,at least in part, to the presence of microscopic particles of UHMWPEproduced during the wear of the UHMWPE components. The reaction of thebody to these particles includes inflammation and deterioration of thetissues, particularly the bone to which the orthopaedic implant isanchored. Eventually, the orthopaedic implant becomes painful and/orloose and must be revised and/or replaced.

In order to increase the useful life of orthopaedic implants havingUHMWPE parts, several attempts have been made to increase the wearresistance of the UHMWPE, thereby decreasing the number of wearparticles that can cause tissue necrosis and/or osteolysis. One methodfor increasing the wear resistance of UHMWPE utilizes exposure tohigh-energy radiation, such as gamma radiation, in an inert orreduced-pressure atmosphere to induce cross-linking between thepolyethylene molecules. This cross-linking creates a three-dimensionalnetwork of polyethylene molecules within the polymer which renders itmore resistant to wear, such as adhesive wear. However, the freeradicals formed upon irradiation of UHMWPE can also participate inoxidation reactions which reduce the molecular weight of the polymer viachain scission, leading to degradation of physical properties,embrittlement, and a significant increase in wear rate. Moreover, thethree-dimensional network produced by the cross-linking reaction canreduce the mechanical properties of the UHMWPE.

There are several processes that have been developed to effectively andefficiently reduce the number of free radicals present in irradiatedUHMWPE, all of which have met with varying degrees of success (see, e.g.U.S. Pat. No. 5,414,049). Moreover, while the cross-linking of theUHMWPE and other known methods can increase the wear resistance of amedical implant or medial implant part comprising UHMWPE, such implantsor implant parts can still produce microscopic wear particles of UHMWPEthat can lead to the eventual failure of the medical implant or medicalimplant part.

A need exists for alternative orthopaedic implants comprising UHMWPE andmethods for producing and using such implants. The invention providessuch an implant and such methods. These and other advantages of theinvention, as well as additional inventive features, will be apparentfrom the description of the invention provided herein.

BRIEF SUMMARY OF THE INVENTION

The invention provides a medical implant or medical implant partcomprising a body and a surface layer, wherein the surface layercomprises a mixture comprising at least one hydrophilic polymer having amelt index of about 0.5 g/10 min or less and ultrahigh molecular weightpolyethylene having a weight average molecular weight of about 400,000atomic mass units or more.

The invention also provides a method for producing a medical implant ormedical implant part comprising a body and a surface layer, the methodcomprising the steps of: (a) providing a compression mold for themedical implant or medical implant part having an internal volume, (b)providing a matrix of ultrahigh molecular weight polyethylene, whereinthe ultrahigh molecular weight polyethylene has a weight averagemolecular weight of about 400,000 atomic mass units or more, (c)dispersing at least one hydrophilic polymer in the matrix of ultrahighmolecular weight polyethylene to produce a mixture comprising at leastone hydrophilic polymer and ultrahigh molecular weight polyethylene, (d)filling at least a portion of the internal volume of the compressionmold with the mixture obtained in step (c), (e) compressing the mixturecontained within the compression mold for a time and under conditionssufficient to form a medical implant or medical implant part therefrom,and (f) removing the medical implant or medical implant part from thecompression mold.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a medical implant or medical implant partcomprising a body and a surface layer, wherein the surface layercomprises a mixture comprising at least one hydrophilic polymer andultrahigh molecular weight polyethylene having a weight averagemolecular weight of about 400,000 atomic mass units or more.

The medical implant or medical implant part of the invention can be anysuitable medical implant or medical implant part. Suitable medicalimplants or medical implant parts include, but are not limited to, theacetabular cup, the insert or liner of the acetabular cup, or trunnionbearings (e.g., between the modular head and the stem) of artificial hipjoints, the tibial plateau, patellar button (patello-femoralarticulation), and trunnion or other bearing components of artificialknee joints, the talar surface (tibiotalar articulation) and otherbearing components of artificial ankle joints, the radio-numeral joint,ulno-humeral joint, and other bearing components of artificial elbowjoints, the glenoro-humeral articulation and other bearing components ofartificial shoulder joints, intervertebral disk replacements and facetjoint replacements for the spine, temporo-mandibular joints (jaw), andfinger joints.

As noted above, the medical implant or medical implant part comprises abody and a surface layer. The body of the medical implant or medicalimplant part can comprise any suitable material (e.g., any biocompatiblematerial). The body of the medical implant or medical implant partdesirably comprises, consists of, or consists essentially of a materialhaving a mechanical strength sufficient to withstand the forces to whichthe medical implant or medical implant part will be subjected during itslifetime. In certain embodiments, the body of the medical implant ormedical implant part comprises ultrahigh molecular weight polyethylene.

The surface layer of the medical implant or medical implant partcomprises a mixture comprising at least one hydrophilic polymer andultrahigh molecular weight polyethylene. The hydrophilic polymer and theultrahigh molecular weight polyethylene preferably are substantiallyuniformly dispersed throughout the mixture. In view of the hydrophobicnature of ultrahigh molecular weight polyethylene, the hydrophilicpolymer and the ultrahigh molecular weight polyethylene typically willbe present in the mixture in two distinct phases. Preferably, themixture resembles an emulsion in which globules of a discontinuousphase, which phase is comprised of the minor component of the matrix(e.g., hydrophilic polymer), are dispersed in a continuous phase, whichphase is comprised of the major component of the matrix (e.g., ultrahighmolecular weight polyethylene).

The surface layer of the medical implant or medical implant part can beprovided in any suitable size and/or shape. Generally, the surface layerof the medical implant or medical implant part has a thickness of about1 mm or more (e.g., about 0.5 mm or more, about 0.75 mm or more, about1.25 mm or more, about 1.5 mm or more, or about 2 mm or more). Incertain embodiments, the surface layer of the medical implant or medicalimplant part has a thickness that extends through substantially all ofthe medical implant or medical implant part. The surface layer of themedical implant or medical implant part can have a thickness thatextends through the entire medical implant or medical implant part,i.e., the body and surface layer are the same. The surface layer of themedical implant or medical implant part generally is coextensive with atleast one surface of the medical implant or medical implant part.Preferably, the surface layer corresponds to an articulating surface ofthe medical implant or medical implant part.

The hydrophilic polymer can have any suitable melt index. Typically, thehydrophilic polymer has a melt index of about 5 g/10 min or less (e.g.,about 4 g/10 min or less, about 3 g/10 min or less, about 2 g/10 min orless, or about 1 g/10 min or less). Preferably, the hydrophilic polymerhas a melt index of about 0.5 g/10 min or less, more preferably about0.45 g/10 min or less, even more preferably about 0.425 g/10 min orless, and most preferably about 0.4 g/10 min or less. The melt index ofthe hydrophilic polymer is determined in accordance with ASTM StandardD1238-88 (entitled, “Flow Rates of Thermoplastics by ExtrusionPlastometer”) using the following conditions: (i) 190° C., (ii), 21.6 kgweight, (iii) 20 cm³ sample, and (iv) 5.5 minute preheat time. Morespecifically, the equipment used to determine the melt index of thehydrophilic polymer (i.e., the plastometer, the cylinder, the die, thepiston, the heater, the thermometer, etc.) is the same as that definedin ASTM Standard D1238-88; however, the conditions under which the meltindex is measured differ from those specified in the aforementionedstandard. In particular, a 20 cm³ sample of the hydrophilic polymer,which sample is provided as a homogeneous powder or pellets, is used todetermine the melt index of the hydrophilic polymer. The 20 cm³ sampleis placed in the barrel of the plastometer while tamping the material toensure that the entire sample is placed in the barrel and at least 90%of the volume of the barrel is filled with the sample. After the sampleis loaded into the barrel of the plastometer, the piston is insertedinto the barrel, and 10.8 kg of weight is placed on the piston. Thetiming of the 5.5 minute preheating cycle is then begun. During thepreheat cycle, the weight on the piston is adjusted so that the sampleextrudes from the barrel at a rate such that the lower two scribedmarker lines on the piston reach the top of the barrel by the end of the5.5 minute preheat cycle (+/−15 seconds). Immediately prior to thecompletion of the preheat cycle, the weight on the piston is increasedto 21.6 kg. Once the lower of the two scribed marker lines on the pistonreaches the top of the barrel, the extrudate is cut from the barrel, anda timed sampling cycle is begun. Typically, the sampling cycle is about1 minute; however, the length of the sampling cycle can be shorter(e.g., 30 seconds) or longer (e.g., 2 minutes) depending on the rate atwhich the sample extrudes from the barrel of the plastometer. At the endof the sampling cycle, the extrudate is cut from the barrel and weighed.The melt index (in g/10 min) is then calculated using the weight of theextrudate and the duration of the sampling cycle.

The hydrophilic polymer can be any suitable hydrophilic polymer.Preferably, the hydrophilic polymer will not degrade at the compressionmolding temperatures typically used for ultrahigh molecular weightpolyurethane and is capable of cross-linking with itself and anotherpolymer (e.g., ultrahigh molecular weight polyethylene) usinghigh-energy radiation (e.g., gamma radiation). The hydrophilic polymerpreferably is a water-soluble, biocompatible polymer. As utilizedherein, the term “biocompatible polymer” is used to refer to any polymerthat is susceptible to implantation in a host (e.g., human host) anddoes not elicit any adverse reactions. Preferably, the hydrophilicpolymer is a water-soluble, biocompatible polymer selected from thegroup consisting of poly(ethylene oxide), polyvinylpyrrolidone,poly(vinyl alcohol), mixtures thereof, and copolymers thereof.

The hydrophilic polymer can be present in the mixture comprising atleast one hydrophilic polymer and ultrahigh molecular weightpolyethylene in any suitable amount. Typically, the hydrophilic polymercomprises about 0.1 wt. % or more, preferably about 0.5 wt. % or more,or more preferably about 1 wt. % or more of the mixture based on thetotal weight of the mixture. The hydrophilic polymer typically comprisesabout 25 wt. % or less, preferably about 20 wt. % or less, or morepreferably about 15 wt. % or less of the mixture based on the totalweight of the mixture. Generally, the hydrophilic polymer comprisesabout 0.1 to about 25 wt. % of the mixture based on the total weight ofthe mixture.

The mixture of the surface layer also comprises ultrahigh molecularweight polyethylene. As utilized herein, the term “ultrahigh molecularweight polyethylene” refers to a polyethylene polymer having a weightaverage molecular weight of about 400,000 atomic mass units or more.Preferably, the ultrahigh molecular weight polyethylene has a weightaverage molecular weight of about 1,000,000 (e.g., about 2,000,000 orabout 3,000,000) atomic mass units or more. Typically, the weightaverage molecular weight of the ultrahigh molecular weight polyethyleneis less than 10,000,000 atomic mass units or less, more preferably about6,000,000 atomic mass units or less. Ultrahigh molecular weightpolyethylene suitable for use in the invention includes, but is notlimited to, commercially available ultrahigh molecular weightpolyethylene, such as GUR 1050 (weight average molecular weight of about5,000,000 to about 6,000,000 atomic mass units) or GUR 1020 (weightaverage molecular weight of about 3,000,000 to about 4,000,000 atomicmass units) powdered ultrahigh molecular weight polyethylene from Ticona(Summit, N.J.). Preferably, the ultrahigh molecular weight polyethylenedoes not contain stabilizers, antioxidants, or other chemical additiveswhich may have potential adverse effects in medical applications.

Preferably, at least a portion of the hydrophilic polymer is covalentlybonded to at least a portion of the ultrahigh molecular weightpolyethylene contained in the surface layer of the medical implant ormedical implant part. The hydrophilic polymer and the ultrahighmolecular weight polyethylene can be covalently bonded to each otherusing any suitable means. For example, the hydrophilic polymer and theultrahigh molecular weight polyethylene can be bonded to each otherusing a suitable chemical cross-linking agent. Preferably, thehydrophilic polymer and the ultrahigh molecular weight polyethylene arecovalently bonded to each other by cross-linking using high-energyirradiation. The hydrophilic polymer and the ultrahigh molecular weightpolyethylene can be irradiated by exposing the mixture comprising atleast one hydrophilic polymer and ultrahigh molecular weightpolyethylene and/or the medical implant or medical implant part to asuitable amount of gamma, x-ray, or electron beam radiation. Preferably,the mixture comprising at least one hydrophilic polymer and ultrahighmolecular weight polyethylene is irradiated by exposing the mixture toabout 0.5 to about 10 Mrad (e.g., about 1.5 to about 6 Mrad) of gammaradiation using methods known in the art. While the mixture can beexposed to amounts of radiation falling outside of the aforementionedrange, such amounts of radiation tend to produce a surface layer withunsatisfactory properties. In particular, radiation doses of less thanabout 0.5 Mrad generally provide insufficient cross-linking of thehydrophilic polymer and the ultrahigh molecular weight polyethylene.Furthermore, while doses of greater than 10 Mrad may be used, theadditional cross-linking that is achieved generally is offset by theincreased brittleness of the surface layer. When the hydrophilic polymerand the ultrahigh molecular weight polyethylene are bonded by exposingthe mixture to high-energy radiation, the mixture preferably isirradiated in an inert or reduced-pressure atmosphere. The free radicalsgenerated in the mixture (i.e., in the hydrophilic polymer and theultrahigh molecular weight polyethylene) preferably are quenchedfollowing the irradiation of the mixture using any suitable method, manyof which are known in the art.

The invention also provides a method for producing a medical implant ormedical implant part comprising a body and a surface layer, the methodcomprising the steps of: (a) providing a compression mold for themedical implant or medical implant part having an internal volume, (b)providing a matrix of ultrahigh molecular weight polyethylene, whereinthe ultrahigh molecular weight polyethylene has a weight averagemolecular weight of about 400,000 atomic mass units or more, (c)dispersing at least one hydrophilic polymer in the matrix of ultrahighmolecular weight polyethylene to produce a mixture comprising at leastone hydrophilic polymer and ultrahigh molecular weight polyethylene, (d)filling at least a portion of the internal volume of the compressionmold with the mixture obtained in step (c), (e) compressing the mixturecontained within the compression mold for a time and under conditionssufficient to form a medical implant or medical implant part therefrom,and (f) removing the medical implant or medical implant part from thecompression mold.

The characteristics of the medical implant or medical implant partproduced by the method of the invention (e.g., the body, the surfacelayer, the hydrophilic polymer, the ultrahigh molecular weightpolyethylene, etc.) can be the same as those set forth above for themedical implant or medical implant part of the invention.

As noted above, the method of the invention comprises providing acompression mold for the medical implant or medical implant part havingan internal volume. The term “compression mold” is utilized herein torefer to a mold typically having two halves which, when joined together,define an internal volume (i.e., mold cavity). The compression mold canbe provided in any suitable configuration. Generally, the compressionmold is configured such that the internal volume of the compression mold(i.e., the mold cavity) defines the medical implant or medical implantpart in a substantially complete form (i.e., in substantially the sameform as will be used for implantation in the host). However, it will beunderstood that the medical implant or medical implant part produced bythe method of the invention can also be subjected to further processing(e.g., machining) to provide the medical implant or medical implant inthe final form used for implantation in the host.

The matrix of ultrahigh molecular weight polyethylene can be provided inany suitable form. Preferably, the matrix of ultrahigh molecular weightpolyethylene comprises, consists essentially of, or consists ofultrahigh molecular weight polyethylene in a powdered or pelletizedform.

At least one hydrophilic polymer is dispersed in the ultrahigh molecularweight polyethylene to produce a mixture (e.g., a substantiallyhomogeneous mixture) comprising at least one hydrophilic polymer andultrahigh molecular weight polyethylene. The hydrophilic polymer can bedispersed in the matrix of ultrahigh molecular weight polyethylene usingany suitable means. Typically, the hydrophilic polymer and the ultrahighmolecular weight polyethylene are provided in a powdered or pelletizedform, and the hydrophilic polymer is dispersed in the ultrahighmolecular weight polyethylene by dry blending the two components to forma mixture comprising the hydrophilic polymer and ultrahigh molecularweight polyethylene.

As noted above, at least a portion of the internal volume of thecompression mold (i.e., mold cavity) is filled with the mixturecomprising, consisting essentially of, or consisting of the hydrophilicpolymer and ultrahigh molecular weight polyethylene. Preferably, theportion of the internal volume of the compression mold filled with themixture comprises a portion of the surface layer of the medical implantor medical implant part. In such an embodiment, the surface layerpreferably corresponds to an articulating surface of the medical implantor medical implant part. In another embodiment, substantially all of theinternal volume of the compression mold preferably is filled with themixture comprising at least one hydrophilic polymer and ultrahighmolecular weight polyethylene. When only a portion of the internalvolume of the compression mold is filled with the mixture, the remainingportion of the internal volume of the compression mold (e.g., theportion of the compression mold corresponding to the body of the medicalimplant or medical implant part) is filled with another suitablematerial, preferably ultrahigh molecular weight polyethylene.

After at least a portion of the internal volume of the compression moldis filled, the mixture comprising at least one hydrophilic polymer andultrahigh molecular weight polyethylene contained within the compressionmold is compressed for a time and under conditions sufficient to form amedical implant or medical implant part therefrom. The mixture iscompressed by any suitable means, such as by mating the two halves ofthe compression mold and applying an external force in a direction suchthat any substance contained within the mold (e.g., the mixturecomprising at least one hydrophilic polymer and ultrahigh molecularweight polyethylene) is subjected to a compressive force. It will befurther understood that the particular time and conditions (e.g., forceapplied to the compression mold) necessary to form a medical implant ormedical implant part will depend upon several factors, such as thecomposition of the mixture (e.g., the type and/or amount of thehydrophilic polymer, and/or the molecular weight of the ultrahighmolecular weight polyethylene), the size (e.g., thickness) of thedesired medical implant or medical implant part, as well as others.

In certain embodiments, the method of the invention further comprisesthe step of cross-linking at least a portion of the hydrophilic polymerand the ultrahigh molecular weight polyethylene. The hydrophilic polymerand the ultrahigh molecular weight polyethylene can be cross-linkedusing any suitable method (e.g., chemical cross-linking or high-energyirradiation). Preferably, the method of the invention further comprises(g) irradiating at least a portion of the mixture comprising at leastone hydrophilic polymer and ultrahigh molecular weight polyethylene fora time and under conditions sufficient to cross-link at least a portionof the hydrophilic polymer and ultrahigh molecular weight polyethylenecontained therein. The mixture comprising at least one hydrophilicpolymer and ultrahigh molecular weight polyethylene can be irradiated atany suitable point, but preferably is irradiated after the completion ofstep (f). The time and conditions sufficient to cross-link at least aportion of the hydrophilic polymer and ultrahigh molecular weight canvary depending upon several factors. For example, the identity of thehydrophilic polymer, the thickness of the medical implant or medicalimplant part, the desired degree of cross-linking, and the desired depthof the cross-linking can impact the time and/or conditions required tocross-link the hydrophilic polymer and ultrahigh molecular weightpolyethylene.

Preferably, the hydrophilic polymer and the ultrahigh molecular weightpolyethylene are cross-linked by exposing the medical implant or medicalimplant part to high-energy radiation. The medical implant or medicalimplant part can be irradiated by exposure to a suitable amount ofgamma, x-ray, or electron beam radiation. Preferably, the medicalimplant or medical implant part is exposed to about 0.5 to about 10 Mrad(e.g., about 1.5 to about 6 Mrad) of gamma radiation using methods knownin the art. While the medical implant or medical implant part can beexposed to amounts of radiation falling outside of the aforementionedrange, such amounts of radiation tend to produce a medical implant ormedical implant part with unsatisfactory properties. In particular,radiation doses of less than about 0.5 Mrad generally provideinsufficient cross-linking of the hydrophilic polymer and the ultrahighmolecular weight polyethylene. Furthermore, while doses of greater than10 Mrad may be used, the additional cross-linking that is achievedgenerally is offset by the increased brittleness of the medical implantor medical implant part.

Preferably, the mixture comprising at least one hydrophilic polymer andultrahigh molecular weigh polyethylene is irradiated in an inert orreduced-pressure atmosphere. Irradiating the mixture comprising at leastone hydrophilic polymer and ultrahigh molecular weigh polyethylene(i.e., non-oxidizing) or reduced-pressure atmosphere reduces the effectsof oxidation and chain scission reactions which can occur duringirradiation in an oxidative atmosphere. Typically, the mixturecomprising at least one hydrophilic polymer and ultrahigh molecularweigh polyethylene (e.g., the medical implant or medical implant part)is placed in an oxygen-impermeable package during the irradiation step.Suitable oxygen-impermeable packaging materials include, but are notlimited to, aluminum, polyester coated metal foil (e.g., the Mylar®product available from DuPont Teijin Films), polyethylene terephthalate,and poly(ethylene vinyl alcohol). In order to further reduce the amountof oxidation which occurs during the irradiation of the mixturecomprising at least one hydrophilic polymer and ultrahigh molecularweigh polyethylene, the oxygen-impermeable packaging may be evacuated(e.g., the pressure within the packaging may be reduced below theambient atmospheric pressure) and/or flushed with an inert gas (e.g.,nitrogen, argon, helium, or mixtures thereof) after the mixturecomprising at least one hydrophilic polymer and ultrahigh molecularweight polyethylene (e.g., the medical implant or medical implant part)has been placed therein.

When at least a portion of the hydrophilic polymer and ultrahighmolecular weight polyethylene are cross-linked by irradiating themixture comprising at least one hydrophilic polymer and ultrahighmolecular weight polyethylene, the method of the invention preferablyfurther comprises (h) quenching a substantial portion of the freeradicals generated in the mixture comprising at least one hydrophilicpolymer and ultrahigh molecular weight polyethylene during theirradiation of the mixture in step (g). The free radicals containedwithin the irradiated portion of the mixture can be quenched using anysuitable method. For example, the free radicals contained within theirradiated portion of the mixture comprising at least one hydrophilicpolymer and ultrahigh molecular weight polyethylene can be quenched byheating the irradiated mixture to a temperature between room temperatureand the melting point of ultrahigh molecular weight polyethylene in anoxygen-reduced, non-reactive atmosphere for a length of time sufficientto reduce the number of free radicals present in the mixture (see, e.g.,U.S. Pat. Nos. 5,414,049, 6,174,934, and 6,228,900). Alternatively, thefree radicals contained within the irradiated portion of the mixturecomprising at least one hydrophilic polymer and ultrahigh molecularweight polyethylene can be quenched by heating the irradiated mixture toa temperature at or above the melting point of ultrahigh molecularweight polyethylene in an oxygen-reduced, non-reactive atmosphere for alength of time sufficient to reduce the number of free radicals presentin the mixture (see, e.g., U.S. Pat. Nos. 6,017,975, 6,228,900,6,242,507, and 6,316,158). Lastly, the free radicals contained withinthe irradiated portion of the mixture comprising at least onehydrophilic polymer and ultrahigh molecular weight polyethylene can bequenched by immersing the irradiated portion of the mixture in anon-polar solvent for a time and under conditions sufficient to quench asubstantial portion of the free radicals contained therein. Theaforementioned process is described more fully in copending U.S. patentapplication Ser. No. 10/609,749.

The method of the invention preferably further comprises the step ofsterilizing the medical implant or medical implant part using anon-irradiative process. The medical implant or medical implant part canbe sterilized at any suitable point, but preferably is sterilized afterthe completion of step (f), (g), or (h). Sterilizing the medical implantor medical implant part using a non-irradiative method avoids theformation of additional free radicals in the ultrahigh molecular weightpolyethylene, which free radicals could undergo oxidative reactionsresulting in the chain scission of the ultrahigh molecular weightpolyethylene. Suitable non-irradiative sterilization techniques include,but are not limited to, gas plasma or ethylene oxide methods known inthe art. For example, the packaged medical implant or packaged medicalimplant part can be sterilized using a PlazLyte® Sterilization System(Abtox, Inc., Mundelein, Ill.) or in accordance with the gas plasmasterilization processes described in U.S. Pat. Nos. 5,413,760 and5,603,895.

The medical implant or medical implant part can be packaged in anysuitable packaging material. Desirably, the packaging material maintainsthe sterility of the medical implant or medical implant part until thepackaging material is breached. If the medical implant or medicalimplant part has not been irradiated or if the medical implant ormedical implant part has been irradiated and a substantial portion ofthe free radicals contained within the medical implant or medicalimplant part have been quenched, the medical implant or medical implantpart will be relatively stable to atmospheric oxidation. Under suchcircumstances, it would not be necessary to package the medical implantor medical implant part in an inert atmosphere and, therefore, themedical implant or medical implant part could be packaged in anair-impermeable or air-permeable packaging material.

The following examples further illustrate the invention but, of course,should not be construed as in any way limiting its scope.

EXAMPLE 1

This example demonstrates the properties of a medical implant or medicalimplant part of the invention. This example further demonstrates theproduction of a medical implant or medical implant part according to themethod of the invention. Approximately 2 g of powdered poly(ethyleneoxide) (WSR-303 available from Dow, Midland, Michigan), which has aweight average molecular weight of approximately 7.0 million atomic massunits and a melt index of less than about 0.5 g/10 min, was dry blendedwith approximately 23 g of powdered ultrahigh molecular weightpolyethylene (GUR 1020 available from Ticona, Summit, N.J.). The mixturecomprised approximately 8 wt. % of poly(ethylene oxide), which isequivalent to approximately 6.4% by volume. The mixture of poly(ethyleneoxide) and ultrahigh molecular weight polyethylene was then placed in acompression mold and compressed under a suitable temperature andpressure until the ultrahigh molecular weight polyethylene consolidatedto produce a composite of the poly(ethylene oxide) and ultrahighmolecular weight polyethylene.

The average contact angle of a drop of water on the surface of thecomposite of poly(ethylene oxide) and ultrahigh molecular weightpolyethylene was then measured and compared to the contact angle of adrop of water on the surface of virgin ultrahigh molecular weightpolyethylene. The contact angle for each sample was determined bydispensing a water drop on the surface of the samples and thenmeasuring, after about 3 to 5 minutes, the angle between the surface ofthe sample and the surface of the water drop at the point of contactbetween the two. The average contact angle for the composite ofpoly(ethylene oxide) and ultrahigh molecular weight polyethylene wasapproximately 85 degrees. The average contact angle for the virginultrahigh molecular weight polyethylene was approximately 90 degrees.

These results demonstrate that the attraction between water and thesurface of a composite comprising a mixture of a hydrophilic polymer andultrahigh molecular weight polyethylene is relatively greater than theattraction between water and the surface of virgin ultrahigh molecularweight polyethylene. More specifically, the reduced contact angle of thecomposite of poly(ethylene oxide) and ultrahigh molecular weightpolyethylene relative to the virgin ultrahigh molecular weightpolyethylene evinces the relatively greater attraction between water andthe surface of the composite of poly(ethylene oxide) and ultrahighmolecular weight polyethylene relative to the surface of virginultrahigh molecular weight polyethylene.

EXAMPLE 2

This example demonstrates the properties of a medical implant or medicalimplant part of the invention. This example also demonstrates theproduction of a medical implant or medical implant part according to themethod of the invention. Approximately 2 g of powdered poly(ethyleneoxide) (WSR-303 available from Dow, Midland, Mich.), which has a weightaverage molecular weight of approximately 7.0 million atomic mass unitsand a melt index of less than 0.5 g/10 min, was dry blended withapproximately 23 g of powdered ultrahigh molecular weight polyethylene(GUR 1020 available from Ticona, Summit, N.J.). The mixture comprisedapproximately 8 wt. % of poly(ethylene oxide), which is equivalent toapproximately 6.4% by volume. The mixture of poly(ethylene oxide) andultrahigh molecular weight polyethylene was then placed in a compressionmold and compressed under a suitable temperature and pressure until theultrahigh molecular weight polyethylene consolidated to produce acomposite of the poly(ethylene oxide) and ultrahigh molecular weightpolyethylene.

The composite of poly(ethylene oxide) and ultrahigh molecular weightpolyethylene was then packaged in aluminum foil, and the air wasevacuated from the foil packaging. The packaged composite was irradiatedby exposing the composite to approximately 50 kGy (approximately 5 Mrad)of gamma radiation and then heated to 140° C. in a vacuum oven for about2 hours to quench a substantial portion of the free radicals produced inthe composite during the irradiation process.

The average contact angle of a drop of water on the surface of theirradiated composite of poly(ethylene oxide) and ultrahigh molecularweight polyethylene was then measured and compared to the contact angleof a drop of water on the surface of virgin ultrahigh molecular weightpolyethylene. The contact angle for each sample was determined bydispensing a water drop on the surface of the samples and thenmeasuring, after about 3 to 5 minutes, the angle between the surface ofthe sample and the surface of the water drop at the point of contactbetween the two. The average contact angle for the irradiated compositeof poly(ethylene oxide) and ultrahigh molecular weight polyethylene wasapproximately 73 degrees. However, the average contact angle for theirradiated composite decreased to approximately 54 degrees after theirradiated composite was soaked in water for about one hour before thecontact angle measurements were taken. The average contact angle for thevirgin ultrahigh molecular weight polyethylene was approximately 90degrees.

These results demonstrate that an irradiated composite comprising amixture of a hydrophilic polymer and ultrahigh molecular weightpolyethylene exhibits improved hydrophilicity relative to virginultrahigh molecular weight polyethylene. More specifically, the reducedcontact angle of the irradiated composite of poly(ethylene oxide) andultrahigh molecular weight polyethylene relative to the virgin ultrahighmolecular weight polyethylene evinces the improved hydrophilicity of theirradiated composite of poly(ethylene oxide) and ultrahigh molecularweight polyethylene relative to the virgin ultrahigh molecular weightpolyethylene. Furthermore, the contact angle measurements taken aftersoaking the irradiated composite in water demonstrates that thehydrophilicity of the irradiated composite improves as the irradiatedcomposite absorbs water from an aqueous environment (e.g., the humanbody).

EXAMPLE 3

This example demonstrates the optional cross-linking of the hydrophilicpolymer and ultrahigh molecular weight polyethylene in a medical implantor medical implant part according to the invention. Three composites(Composite 3A, 3B, and 3C) comprising a mixture of a hydrophilic polymer(i.e., poly(ethylene oxide)) and ultrahigh molecular weight polyethylenewere prepared in accordance with the procedures set forth in Example 1(Composite 3A) and Example 2 (Composite 3C). Composite 3B was generallyprepared in accordance with the procedure set forth in Example 2, butthe composite was not subjected to the quench process following gammairradiation. Each composite was then submerged in a sonicated water bathfor approximately 24 hours at a temperature of approximately 65° C. Thecomposites then were removed from the water bath, dried, and weighed todetermine the change in weight due to submersion in the water bath. Theresults of the measurements are summarized below in Table 1. TABLE 1Irradiation Doses, Quench Particulars, and Weight Loss for Composites3A-3C. Weight Loss (wt. % based Com- Radiation on initial weight ofposite Dose Quench poly(ethylene) oxide) 3A — — 17 3B 50 kGy (5 Mrad) —5.6 3C 50 kGy (5 Mrad) 2 hours No detectable weight loss at 140° C.

As evidenced by the data set forth in Table 1, the hydrophilic polymerand the ultrahigh molecular weight polyethylene of the composite can becross-linked by irradiating the composite. In particular, the differentweight loss for each composite suggests that the hydrophilic polymer andultrahigh molecular weight polyethylene in each composite are associatedin different ways and/or to different degrees. For example, Composite3A, which was not irradiated, lost approximately 17 wt. % of the initialpoly(ethylene oxide) present in the composite after submersion in thewater bath. By way of contrast, Composites 3B and 3C, which were bothirradiated, lost only approximately 5.6 wt. % and no detectable amount,respectively, of the initial poly(ethylene oxide) present in thecomposite after submersion in the water bath. Insofar as thepoly(ethylene oxide) is the only water-soluble component present in thecomposites, the observed weight loss for Composites 3A and 3B must besolely attributable to the dissolution of the poly(ethylene oxide) atthe surface of the composites. Furthermore, the free radicals producedin the ultrahigh molecular weight polyethylene and hydrophilic polymerduring the irradiation process would, at least in part, interact witheach other and combine to produce cross-links between the two polymers.Thus, this data indirectly demonstrates that the hydrophilic polymer(i.e., poly(ethylene oxide)) has cross-linked with the ultrahighmolecular weight polyethylene, thereby preventing the hydrophilicpolymer from dissolving in the water bath.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

1. A medical implant or medical implant part comprising a body and asurface layer, wherein the surface layer comprises a mixture comprisingat least one hydrophilic polymer having a melt index of about 0.5 g/10min or less and ultrahigh molecular weight polyethylene having a weightaverage molecular weight of about 400,000 atomic mass units or more. 2.The medical implant or medical implant part of claim 1, wherein theultrahigh molecular weight polyethylene has a weight average molecularweight of about 1,000,000 atomic mass units or more.
 3. The medicalimplant or medical implant part of claim 1, wherein at least a portionof the hydrophilic polymer is covalently bonded to at least a portion ofthe ultrahigh molecular weight polyethylene.
 4. The medical implant ormedical implant part of claim 1, wherein the hydrophilic polymer is awater-soluble, biocompatible polymer.
 5. The medical implant or medicalimplant part of claim 4, wherein the hydrophilic polymer is awater-soluble, biocompatible polymer selected from the group consistingof poly(ethylene oxide), polyvinylpyrrolidone, poly(vinyl alcohol),mixtures thereof, and copolymers thereof.
 6. The medical implant ormedical implant part of claim 4, wherein the hydrophilic polymercomprises about 0.1 to about 25 wt. % of the mixture comprising at leastone hydrophilic polymer and ultrahigh molecular weight polyethylene. 7.The medical implant or medical implant part of claim 1, wherein thesurface layer has a thickness of about 1 mm or more.
 8. The medicalimplant or medical implant part of claim 1, wherein the body comprisesultrahigh molecular weight polyethylene.
 9. The medical implant ormedical implant part of claim 1, wherein the body is composed of thesame materials as the surface layer.
 10. The medical implant or medicalimplant part of claim 1, wherein the surface layer corresponds to anarticulating surface of the medical implant or medical implant part. 11.A method for producing a medical implant or medical implant partcomprising a body and a surface layer, the method comprising the stepsof: (a) providing a compression mold for the medical implant or medicalimplant part having an internal volume, (b) providing a matrix ofultrahigh molecular weight polyethylene, wherein the ultrahigh molecularweight polyethylene has a weight average molecular weight of about400,000 atomic mass units or more, (c) dispersing at least onehydrophilic polymer in the matrix of ultrahigh molecular weightpolyethylene to produce a mixture comprising at least one hydrophilicpolymer and ultrahigh molecular weight polyethylene, (d) filling atleast a portion of the internal volume of the compression mold with themixture obtained in step (c), (e) compressing the mixture containedwithin the compression mold for a time and under conditions sufficientto form a medical implant or medical implant part therefrom, and (f)removing the medical implant or medical implant part from thecompression mold.
 12. The method of claim 11, wherein the portion of theinternal volume of the compression mold filled with the mixturecomprising at least one hydrophilic polymer and ultrahigh molecularweight polyethylene comprises a portion of the surface layer of themedical implant or medical implant part, and the surface layercorresponds to an articulating surface of the medical implant or medicalimplant part.
 13. The method of claim 12, wherein the surface layer hasa thickness of about 1 mm or more.
 14. The method of claim 13, whereinthe portion of the internal volume of the compression mold correspondingto the body of the medical implant or medical implant part is filledwith ultrahigh molecular weight polyethylene.
 15. The method of claim11, wherein substantially all of the internal volume of the compressionmold is filled with the mixture comprising at least one hydrophilicpolymer and ultrahigh molecular weight polyethylene.
 16. The method ofclaim 11, wherein the method further comprises: (g) irradiating at leasta portion of the mixture comprising at least one hydrophilic polymer andultrahigh molecular weight polyethylene for a time and under conditionssufficient to cross-link at least a portion of the hydrophilic polymerand ultrahigh molecular weight polyethylene contained therein.
 17. Themethod of claim 16, wherein the method further comprises: (h) quenchinga substantial portion of free radicals generated in the mixturecomprising at least one hydrophilic polymer and ultrahigh molecularweight polyethylene during the irradiation of the mixture in step (g).18. The method of claim 17, wherein the method further comprises: (i)sterilizing the medical implant or medical implant part using anon-irradiative process, and (j) packaging the medical implant ormedical implant part.
 19. The method of claim 11, wherein the ultrahighmolecular weight polyethylene has a weight average molecular weight ofabout 1,000,000 atomic mass units or more.
 20. The method of claim 11,wherein the hydrophilic polymer is a water-soluble, biocompatiblepolymer.
 21. The method of claim 20, wherein the hydrophilic polymer isa water-soluble, biocompatible polymer selected from the groupconsisting of poly(ethylene oxide), polyvinylpyrrolidone, poly(vinylalcohol), mixtures thereof, and copolymers thereof.
 22. The method ofclaim 11, wherein the hydrophilic polymer comprises about 0.1 to about25 wt. % of the mixture comprising at least one hydrophilic polymer andultrahigh molecular weight polyethylene.